The present invention relates to a turbine or energy converting apparatus rotatable in only one direction in a reciprocating air or water flow obtainable by wind or wave power independently of the direction of the flow.
As to apparatus of this type, there has been known a turbine such as shown in FIG. 1. In FIG. 1(a), which is a front view of the conventional turbine, reference numeral 1 denotes a rotor blade and 2 denotes a rotary shaft. FIG. 1(b) is a cross-section of the rotor blade 1 along the line 1b-1b in FIG. 1(a). The rotor blade 1 has a cross-section such as is generally known as a symmetrical blade form in the field of aerodynamics. Such a proposal can be found, for example in Japanese Patent Application Laid-Open No. 92060/1978.
In FIG. 2, it is assumed that the rotor blade 1 shown in section is moving with a velocity U as indicated in the drawing. Assuming that a flow comes in with a velocity V substantially perpendicularly to the moving direction of the rotor blade 1, the relative velocity of the flow with respect to the rotor blade can be expressed by W, as indicated in FIG. 2 on the velocity triangle. From hydrodynamics, it is known that the relative velocity W produces dyamic lift L in the rotor blade perpendicularly to the relative velocity W, and reaction (drag) in the same direction as W. The flow force Ft along the travelling direction t to the rotor blade is expressed as follows, on the basis of the geometrical relationships. EQU Ft=L sin .alpha.-D cos .alpha.
where, .alpha. represents the angle of elevation, which is the angle formed between the travelling direction and the relative velocity direction of the rotor blade, and is therefore expressed by .alpha.=tan.sup.-1 (V/U). If the angle of elevation .alpha. is properly selected, it is possible to make Ft&gt;0 and therefore it is possible to impart a driving force to the rotor blade in the direction t, under the condition of the existence of a flow.
Accordingly, in FIG. 1, if there exists a flow in the direction parallel to the rotary shaft 2, the flow force at the respective radii of the rotor blade 1 will become as shown in FIG. 2, so that the rotor blade 1 is enabled to rotate about the rotary shaft 2. Further, since a symmetrical blade form is employed for the rotor blade 1, it is possible to make the direction of the flow force Ft acting on the rotor blade 1 unchanged, independently of the direction of the flow parallel to the rotary shaft 2, from which directions the flow comes in. Therefore, it is possible to cause the rotor blade 1 to rotate in one direction in a reciprocating flow obtained, for example, by wave energy.
The conventional apparatus is arranged in the manner described above, and with respect to the rotor blade 1 per se, as shown in FIG. 3, since a uniform flow comes in to the rotor blade 1 with a velocity V, the peripheral speed of the rotor blade 1 is higher at the tip portion while lower at the hub portion. Accordingly, the elevation angle of the relative velocity W with respect to the rotor blade 1 changes at the respective radial positions of the rotor blade 1. In view of the characteristic of the blade, the fact that the elevation angle changes at the respective radial positions of the rotor blade 1 in spite of the existance of an optimum elevation angle means that it becomes impossible to operate at the optimum point, and it is essentially impossible to efficiently absorb energy in such a turbine. For example, in FIG. 3(a), assuming that the elevation angle is represented by .alpha..sub.t when tip peripheral velocity, the flow velocity and the relative velocity are represented by U.sub.t, V and W.sub.t respectively, the elevation angle .alpha..sub.t is expressed by the following equation: EQU .alpha..sub.t =tan.sup.-1 (V/U.sub.t).
Further assume that this elevation angle .alpha..sub.t is the optimum one in view of the form of the blade. In FIG. 3(b), at the hub portion, the elevation angle .alpha..sub.h is expressed by the following equation: EQU .alpha..sub.h =tan.sup.-1 (V/U.sub.h).
Thus, EQU U.sub.t /U.sub.h =.gamma..sub.t /.gamma..sub.h (&gt;1)
where .gamma., .gamma..sub.t, and .gamma..sub.h represent the rotor radius, the rotor radius at the tip portion and rotor radius at the hub portion, respectively.
Thus, naturally, EQU .alpha..sub.t &gt;.alpha..sub.h,
and when the ratio of the tip to hub radius is large, the elevation angle .alpha..sub.h becomes large so that the elevation angle may naturally cause the blade to enter a stalling range. As a matter of course, it is impossible to expect high efficiency in an arrangement which has a partial stalling range.
Further, as will be appreciated from FIG. 1, the rigidity changes largely in the radial direction at the respective radius positions and therefore it becomes difficult to make an evaluation of turbine performance, resulting in a design problem. This means that optimum rigidity can be selected only at a certain radii, creating a further impediment against obtaining high efficiency.